The SAS task catcorr is run over the unique EPIC source list to
update the sky coordinates by cross-matching them with up to three reference
external catalogues, (i) the USNO B1 catalogue [1]}, (ii)
the 2MASS catalogue [2]}, and (iii) the SDSS (DR9)
catalogue [3]}, to find optical or infrared (IR)
counterparts and to find the small frame shifts or rotations that optimise
the match. If sufficient matches are found within the FOV to optimise the
correction, these shifts are then applied to the positions of all the EPIC
sources in the field. The average 1\(\sigma \)  field correction is of the order
of 2.5. Where catcorr fails to obtain a statistically reliable
result, the associated systematic error is assumed to be 1.5.
